In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks and the thickness of the recording material film are smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a storage plane actually has a physical dimension in the depth direction, too, the term “storage plane (information plane)” will be replaced herein by another term “information layer”. Every optical disc has at least one such information layer. Optionally, a single information layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on an optical disc or to write data on a rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position (or focal point) of the light beam is always located on the information layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
To get the focus control and tracking control done, a focus error or a tracking error needs to be detected based on the light reflected from an optical disc and the location of the light beam spot needs to be adjusted so as to minimize the error. The magnitudes of the focus error and the tracking error are respectively represented by a “focus error (FE) signal” and a “tracking error (TE) signal” that are generated based on the light reflected from the optical disc.
However, if there is any defect such as dust or a scratch on the surface of an optical disc, the intensity of the reflected light will decrease significantly when the light beam passes through that defect. As a result, a TE signal with an appropriate level cannot be generated anymore, thus producing tracking abnormality. Consequently, some type of tracking failure such as a track jump would happen without taking any countermeasure. To avoid such tracking failures, when the presence of any defect is sensed by such a decrease in the intensity of the reflected light, a tracking control signal is temporarily put on hold at the level just before the defect according to a conventional technique. As long as the tracking control signal is held at a certain level, even if any abnormality occurred on the TE signal, the tracking control would not be affected by such abnormality. As a result, tracking failures would hardly occur due to the defects.
Once the light beam has passed the defect, the tracking control signal is preferably taken out of hold as quickly as possible. That is to say, the tracking control signal is put on hold only while the intensity of the reflected light becomes lower than a preset level (i.e., reference sensing level) due to the presence of the defect.
Such an optical disc apparatus that is designed to avoid tracking failures by sensing defects on an optical disc is disclosed in Patent Documents Nos. 1 and 2, for example.    Patent Document No. 1: Japanese Patent Publication No. 2912251    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2003-162834